The problem of scale formation and its attendant affects have troubled water systems for many years. For instance, scale tends to accumulate on the internal walls of various water systems, such as boiler and cooling water systems, thereby reducing heat transfer properties and fluid flow through heat exchanger tubes.
One particular type of deposit, silica, is especially troublesome in some systems. Where the water used in cooling the systems and water cooled industrial heat exchangers is taken directly from lakes, rivers, ponds, or municipal water sources, various amounts of dissolved and suspended solids, including silica, are present. Problems are compounded in operations where water is concentrated, or cycled up, during the process, e.g., cooling systems. As the water evaporates, the silica concentrations increase increasing both the occurrence and the degree of deposition. Increasing silica concentrations can also result in monomeric silica undergoing polymerization to form gelular and/or colloidal silica which can form tenacious deposits.
In cooling water systems, silica and silicate compounds form deposits on the internal metal surfaces in contact with the water flowing through the system. In this manner, heat transfer efficiency becomes severely impeded, which in turn has a deleterious effect on the overall operating efficiency of the cooling water system. Silica and silicate deposition also causes problems on other conduit and piping surfaces, as well as on equipment such as valves, nozzles and pumps.
Although current industrial cooling systems make use of sophisticated external treatments of the feed water, e.g., coagulation, filtration, softening of water prior to it being fed into the water system, these operations are only moderately effective. In all cases, external treatment does not in itself provide adequate treatment since muds, sludge, silts, and dissolved solids such as silicate can escape the treatment and eventually are introduced into the system.
Silica (silicon dioxide) appears naturally in a number of crystalline and amorphous forms, all of which are sparingly soluble in water; thus leading to the formation of undesirable deposits. Silicates are salts derived from silica or the silicic acids, especially orthosilicates and metosilicates, which may combine to form polysilicates. All of these, except the alkali silicates are sparingly soluble in water. A number of different forms of silica and silicate salt deposits are possible, and formation depends, among other factors, on the temperature and pH of the water.
Various methods have been utilized for resolving the problem of sludge and silt, including silica, deposition. U.S. Pat. No. 5,378,368 discloses the use of polyether polyamino methylene phosphonates to control silica/silicate deposition in industrial water systems. The polyether polyamino methylene phosphonates may be used alone or in combination with a polymer additive.
U.S. Pat. No. 5,078,879 discloses the use of 2-phosphono-butane tricarboxylicate acid-1,2,4 alone or preferably in combination with an anionic polymer such as a carboxylic/sulfonic polymer, to control the formation of silica/silicate deposits in aqueous systems.
U.S. Pat. No. 4,933,090 discloses the use of a select phosphonate and optionally a carboxylic/sulphonic/polyalkylene oxide polymer to control silica/silicate deposition.
U.S. Pat. No. 4,874,527 discloses the use of an imine polymer, a phosphanate and optionally a source of molybdate or borate ions to control the formation of silica/silicate deposits in aqueous systems.
U.S. Pat. No. 5,158,685 discloses the use of a combination of a hydroxyphosphonoacetic acid and an acrylic acid/alyl hydroxy propyl sulphonate ether polymer to control silica/silicate deposition in cooling water systems.
U.S. Pat. No. 5,300,231 discloses the use of polyether polyamino methylene phosphonates in combination with hydroxy-phosphono acetic acid or amino tris methylene phosphonate to control silica/silicate deposition in various industrial water systems.
U.S. Pat. No. 4,405,461 discloses the use of a treatment comprising an amine to which is attached at least a pair of terminal groups selected from furfuryl and saturated or unsaturated hydrocarbon radicals substituted with one or more hydroxy and carboxy, or a hydrohalide thereof to control the deposition of silica-containing scales.